"Les terminales" or "les étudiants en terminale" corresponds to last year students. "En coeur" means something like "together". It is often used in the expression "chanter en coeur", which means "to sing a song with other people".

It could not be used in another context. For example, "danser en coeur" (to dance) would not make any sense. It has to have something to do with people using their voice.

Yudan Taiteki wrote:So is "en coeur" = "by heart" or "in unison"? "Responding by heart" still doesn't really seem to make that much sense but it's better than before, I guess "in unison" is more sensible.